You will only get the stoned affect and a "You are holding a banned item" message when holding the item via left click and dragging it out from your inventory. You can open treasure bags while they are in your inventory by right clicking on them. You can also continue to right click and open them even when the stoned affect is active. With this someone could potentially spam thousands of items onto the ground in a matter of seconds, causing a great deal of lag. I found this after someone had dropped treasure bag 3324, and I tried opening them.